Visual notification appliances are widely used in fire alarm systems, and are used for sending visible alarm indications, e.g., flashes, to the field staff in case of emergency. Generally, a visual notification appliance (e.g., a light alarm) is connected to a control panel via a field line or a wireless link. When an emergency situation (e.g., when fire occurs) has been detected, the control panel can trigger a light alarm via the field line, so that the light alarm emits an alarm light to warn the field staff to evacuate in time.
Usually, the light alarm can be mounted on a ceiling (“top-mounted” for short), or mounted on a wall (“wall-mounted” for short). FIGS. 1 and 2 respectively schematically show schematic diagrams of top-mounted and wall-mounted light alarms 100. As shown in FIG. 1, the light alarm 100 typically comprises a base 110, an optical source (not shown) arranged on the base, and a transparent cover 120 covering the optical source. The transparent cover 120 may also function partially as a lens. The light alarm 100 is generally considered to be as a whole, and has a base plane and an optical axis perpendicular to the base plane, the light emergent in the direction of the optical axis having the highest light intensity. In the example shown in FIG. 1, the light alarm 100 is mounted on a ceiling or a roof 64. Thus, the base plane of the light alarm 100 is parallel to the ceiling 64, i.e., as shown in FIG. 1, having an X direction and a Y direction perpendicular to each other. The optical axis is perpendicular to the base plane, and extends in the Z direction as shown in FIG. 1. FIG. 2 shows the case of the light alarm 100 mounted on a wall. In this case, the base plane of the light alarm 100 as shown in FIG. 2 is parallel to a mounting surface, i.e., a wall surface 66. The X direction and Y direction in the base plane are in a plane parallel to the wall surface 66, and the Z direction serving as an optical axis is perpendicular to the wall surface 66.
At present, the fire-protection standards of some nations propose different light intensity distribution requirements for top-mounting and wall-mounting respectively. FIGS. 3A and 3B schematically show two light intensity distribution requirements. Specifically, for top-mounting, emergent light from the light alarm 100 needs to satisfy the light intensity distribution curve as shown in FIG. 3A at least on the X-Z plane and the Y-Z plane as shown in FIG. 1. It can be seen from FIG. 3A that, both on the X-Z plane and on the Y-Z plane, the light intensity distribution needs to satisfy the requirement that a certain light intensity distribution can be achieved within a range of −90° to +90° of the included angle with respect to the Z direction. It is especially noted that, at the positions of +90° and −90° with respect to the Z direction, the light intensity needs to be at least 25%, for example, of the center light intensity. For wall-mounting, emergent light from the light alarm 100 needs to satisfy the light intensity distribution curve as shown in FIG. 3A on the Y-Z plane as shown in FIG. 2, i.e., the light intensity needs to satisfy the curve of FIG. 3A in the range of −90° to 90° of the included angle with respect to the Z direction. Moreover, the emergent light from the light alarm 100 needs to further satisfy the distribution curve as shown in FIG. 3B on the X-Z plane, i.e., the light intensity needs to satisfy the light intensity distribution curve as shown in FIG. 3B in the range of 0° to 90° of the included angle with respect to the Z direction.
As described above, top-mounting and wall-mounting have different requirements of light intensity distribution. In order to satisfy different requirements for light intensity distribution, manufacturers generally design different light alarms for different mounting conditions.